1. Field of the Invention
The invention concerns a processor controlling a rotating machine, a servocontrol system for implementing said method and a rotating machine provided with a system of this kind.
To be more precise, the invention concerns a method of controlling the electromagnetic torque and the stator flux of an asynchronous rotating machine having a high dynamic range from low speeds to high speeds.
2. Description of the Prior Art
U.S. Pat. No. 4,678,248 concerns a control method in which the control parameters are the electromagnetic torque and the stator flux.
The method uses vector modeling of the machine and of the voltage inverter.
The electromagnetic torque of the machine is known to be a function of the angle between the rotor flux rotating vector and the stator flux rotating vector and the moduli of these flux vectors. In other words, the electromagnetic torque .GAMMA.em is a function of the vector product of the rotating flux vectors: EQU .GAMMA..sub.em =K (.phi..sub.R .times..phi..sub.s)
The stator voltage vector V.sub.s is delivered by a three-phase voltage inverter, each phase including a two-state SP2LL (Single Pole 2 Logic Levels) switch. Accordingly, the stator voltage vector V.sub.s can assume eight states V.sub.1 . . . V.sub.8 (2.sup.3), of which two V.sub.1, V.sub.8 are of null amplitude (null states) in the stator fixed frame of reference (.alpha., .beta.), according to the combination of the three SP2LL switches of the inverter.
The DTC (Direct Torque Control) system relies on maintaining the modulus .vertline..phi..sub.s .vertline. of the stator flux rotating vector .phi..sub.s in a hysteresis band H in the stator frame of reference (.alpha., .beta.) and on controlling the torque .GAMMA..sub.em by accelerating the stator flux rotating vector .phi..sub.s relative to the rotor flux .phi..sub.R to increase the torque .GAMMA..sub.em (increase the angle between the two flux vectors) and by stopping the stator flux vector .phi..sub.x so that the rotor flux vector .phi..sub.R catches up with it to reduce the torque .GAMMA..sub.em (reducing the angle between the two flux vectors).
The stator flux vector .phi..sub.s is controlled by means of a finite table. This table contains, for a given location N.sub.i (i=1 . . . 6) of the stator flux vector .phi..sub.s rotating in the plane in the stator (.alpha., .beta.), the states V.sub.1 . . . V.sub.8 of the stator phase voltage vector V.sub.s which enable the stator flux vector to be stopped (null states V.sub.1, V.sub.8) and those for opening the angle between the flux vectors .phi..sub.s, .phi..sub.R whilst maintaining the stator flux vector .phi..sub.s in the hysteresis band H.
In the case of low rotor rotation speeds, the response dynamic of the above solution is very poor. In particular, the negative step response time is in the order of four times the response time of a positive step of the same amplitude.
Furthermore, the proposed technique dedicates control of the stator flux (maintenance of the stator flux modulus in the hysteresis band) to torque control. Configurations in which stator flux control is required concomitantly with control of the torque .GAMMA..sub.em are not provided for.
U.S. Pat. No. 5,610,485 concerns an asynchronous rotating machine control method using the DTC method for one range of speeds and adding hysteresis to the torque. Moreover, the method provides two additional modes of operation, one for low speeds and the other for high speeds.
The mode of operation at low speeds is based on the imposition of an inverter switching frequency.
The mode of operation at high speeds is the full wave mode.
One of the major disadvantages of the above methods results from the fact that switching from one phase vector state to another phase vector state is effected at sampling times when the control system registers overshooting of one of the hystereses. Accordingly, for the system to have a good dynamic (to prevent an excessive overshoot) it is necessary to use very short sampling periods (T.sub.ech =50 .mu.s; f.sub.ech =20 kHz) leading to high sampling frequencies that are significantly higher than the sampling frequencies generally used in real time devices.
Furthermore, the problem of stator flux control (maintaining the stator flux modulus in the hysteresis band) dedicated to torque control has not be solved. Configurations in which stator flux control is required concomitantly with control of the torque .GAMMA..sub.em are still not provided for.
Finally, the change to full wave mode is not simple to effect.
One aim of the present invention is to propose a method of controlling the electromagnetic torque and the stator flux of an asynchronous rotating machine in which the inverter switching times are predicted by calculation and carried out asynchronously with the sampling or calculation times. In this way the sampling frequencies no longer need to be as high as in the prior art and are reduced to the frequencies used as standard in prior art real time devices (between 2 kHz and 5 kHz).
Another aim of the present invention is to propose a method of controlling the electromagnetic torque and the stator flux of an asynchronous rotating machine in which the electromagnetic torque and the stator flux can be regulated concomitantly.
Another aim of the present invention is to propose a method of controlling the electromagnetic torque and the stator flux of an asynchronous rotating machine in which the change to full wave mode does not require any change of strategy.